Friday, January 26, 2007

Eyeglasses and the James Webb Space Telescope

It seems that eyeglasses are an invention of the Middle Ages. At that time, however, glasses were not made of glass, but from a mineral which comes in a clear, transparent crystalline variety. This material, beryl, was the material the first lenses were carved from.

Beryl is, chemically speaking, a cyclosilicate, a compound of the light metals beryllium and aluminium with silicon and oxygen. Its chemical formula is Be3Al2(SiO3)6. In its pure form, beryl is a colourless, clear mineral, but if it is "doped" with trace amounts of other metals, it can have all kinds of colours - and be very precious. Emerald, for example, is a variety of beryl, coloured green by impurities of chromium and, sometimes, iron.

The JWST is a planned satellite telescope which is sometimes called the successor of the Hubble telescope. One of its scopes is to study the first galaxies in the young Universe at a redshift in the range of 5-10. This means that all visible light is shifted far into the infrared. So, the JWST is optimised for observations at the infrared part of the spectrum. It will have a primary mirror with a diameter of 6.5 meters (21 feet 4 inches) which consists of several segments. And these segments will be made of - beryllium.

The primary mirror of the James Webb Space Telescope will consist of 18 honeycomb-shaped segments made of beryllium (Source: NASA)

Metallic beryllium was chosen to produce the mirror because its stiff, light weight, has very small thermal expansion over a wide temperature range and holds its shape at the low 50 Kelvin at which the telescope will operate.

It's a funny coincidence that the same element, which as a main component in a mineral was used to produce the first eyeglasses, will soon help us to look back into the youth of our Universe!

Hi Stefan, curious thingwhen we talk of eyeglassesit always reminds me of a filmZulu - I thinkWhere the African leader (who we'll presume had 'normal' sight) meets a white man wearing spectacles, takes them off him - has a look thru them, and states"this man doesn't want to see the 'real' world"

Of course it is a metaphor - but his comment was triggered by his failure to understand that someone elses eyesight could literally be different from his - and that someone looking through the glasses to 'rectify' sight can presumably see things the same as he did.

Anyway all that aside, people often aske me where Time goes, and someone asked me where does light go ... well JWST may not answer that, but it will be looking at where it comes from - and should be able to say where the light that reached it went

Which leads me to wonder - is there some international agreement on how to avoid clogging up the Lagrange points with space junk? I think there is a protocol for geostationary satellites, and what has to happen at end-of-life.

Hi Bee, definitely a moviejust before the Zulu defeated the British redcoats - like Custer's last stand when the cavalry got slaughtered by the Indians - and when white men, rifles and spectacles (or monocles) were rare curiosities in Africa - and africans still ran around in loin clothes and hunted food with spears

And long long before UV sunglasses or space telescopes and particle physics.

So Bee, photons that reach our eyes get converted to chemical impulses (reactions) in the brain. But where do the other photons go? and where do photons go when no one is looking at the screen, or we turn off the light bulb, and the screen on the tv or laptop

What 'propels' neutrinos, or are they like dust - simply floating around, waiting to hitch a ride

Arun, it's quite unlikely that you'd get Be metal dust from beryl, which is only about 4% Be, the rest being an alumino-silicate. The metal is similar to aluminum in its attraction to oxygen. The problem with the dust will be from the mineral dust itself, which is bad enough, but not as bad as Be metal

thanks for the links and information about the etymology of Brille and beryl!

Is there some international agreement on how to avoid clogging up the Lagrange points with space junk?

I don't know... But then, all those satellites are not located exactly at the Lagrange point, which is unstable, but in a stable orbit around it. Btw, according to the ESA site, besides the James Webb telescope, the ESA satellites Herschel, Planck and Gaia sould be located there. So, probably yes, there will better be a plan where to insert them in orbit around L2 ;-)

About the dangers of beryl and beryllium: the JWST mirror will be made of metallic beryllium - but according to this wikipedia entry, it is used in the defense and aerospace industries as light-weight structural materials in high-speed aircraft, missiles, space vehicles and communication satellites, so I guess there are plenty of safety measures about how to handle it to avoid its hazards.

As Carl says, dust from grinding beryl minerals may be unhealthy for general reasons - Silicosis was very common among the miners in my native Saarland - not so much because of the 10 percent of beryllium atoms it contains (5 percent is the percentage in mass...)

Is there any truth to the rumor that the original NASA design had heptagons? They were redesigned to pentagons for side conservation, then a compromise was hammered out.

(The two extra sides/polygon were to be recycled into T-squares with 80-degree angles. The angle savings thereform were then summed into gyroscope replacements for ISS FUBAR, with pi=3 for diversity hire engineers).